1. Field of the Invention
The present invention relates to an ultrasonic transmitting/receiving device and a method for fabricating the same.
2. Description of the Related Art
A typical known ultrasonic transmitting/receiving device has two ultrasonic transducers that face each other with a predetermined gap therebetween. Each of the ultrasonic transducers includes a piezoelectric vibrator having two electrodes which are separated by a piezoelectric body composed of polarized ceramic material such as lead zirconate titanate (PZT). The surface of one electrode is covered by a backing composed of rubber such as silicone rubber (see Japanese Unexamined Patent Application Publication Nos. 2000-14672, 11-155859, and 6-90950).
In the ultrasonic transducer, application of an electrical signal between the two electrodes excites the piezoelectric vibrator, which then emits ultrasonic waves, whereas reception of ultrasonic waves between the two electrodes vibrates the piezoelectric vibrator, which then converts the vibration into an electrical signal and outputs the signal. The backing absorbs and attenuates the ultrasonic waves emitted through the back surface of the piezoelectric vibrator during vibration.
In measuring the sonic velocity in a liquid medium using the ultrasonic transmitting/receiving device having a pair of ultrasonic transducers, ultrasonic waves are transmitted from one ultrasonic transducer through the liquid medium to the other ultrasonic transducer and the gap transit time, i.e., the time required for the ultrasonic waves to travel from one ultrasonic transducer to the other ultrasonic transducer is measured. The sonic velocity is calculated based on the gap transit time and the distance between the two ultrasonic transducers (see Japanese Unexamined Patent Application Publication Nos. 6-288993 and 6-288990).
When the sonic velocity of a small amount of liquid medium is measured with the ultrasonic transmitting/receiving device having two ultrasonic transducers facing each other, the ultrasonic transducers must be placed at a small distance from each other, resulting in a short gap transit time.
When a backing is provided on the piezoelectric vibrator, the backing attenuates the ultrasonic waves emitted through the back surface of the piezoelectric vibrator. Unfortunately, the acoustic characteristic impedance of the piezoelectric body that defines the piezoelectric vibrator and that of the backing are significantly different. For example, the acoustic characteristic impedance of the piezoelectric body is 30×106 kg/m2·s, whereas that of the backing is 5-7.5×106 kg/m2·s. This great difference in the acoustic characteristic impedance causes ringing. Ringing is a phenomenon in which sonic waves are reflected at the interface between the piezoelectric vibrator and the backing during vibration of the piezoelectric vibrator, generating resonance in the piezoelectric body. Due to this resonance, the piezoelectric vibrator does not stop vibrating in a short period of time but continues vibrating.
If ringing occurs in an ultrasonic transmitting/receiving device having ultrasonic transducers disposed with a small gap, vibration attributed to the ringing may be included in the electrical signal outputted from the receiving ultrasonic transducer. This increases measurement error, thereby deteriorating the accuracy of the time resolution. Specifically, when the sonic velocity of a liquid medium is measured by consecutively emitting ultrasonic waves a number of times at certain intervals and calculating the mean of the obtained times, the ringing influences the accuracy of the measurement.
As described above, when the ultrasonic transmitting/receiving device has a pair of ultrasonic transducers, it is difficult to effectively suppress the ringing during emission of ultrasonic waves. Thus, this type of ultrasonic transmitting/receiving device is unable to maximize time resolution in measuring the sonic velocity.